Add support polymorphic associations

Following up with https://github.com/rails-api/active_model_serializers/pull/367 and https://github.com/rails-api/active_model_serializers/pull/376 I added support for the polymorphic has_one and has_many associations starting out from the current master branch.

Here there is a few different ways we can choose from when serializing the polymorphic models. I choose the one one that looks best but I am open to changes.

## Embedding Polymorphic Associations


Because we need both the id and the type to be able to identify a polymorphic associated model, polymorphic associations are serialized in a slightly different format than common ones.

When embedding entire objects:

```ruby
class PostSerializer < ActiveModel::Serializer
  attributes :id, :title
  has_many :attachments, polymorphic: true
end
```

```json
{
  "post": {
    "id": 1,
    "title": "New post",
    "attachments": [
      {
        "type": "image"
        "image": {
          "id": 3
          "name": "logo"
          "url": "http://images.com/logo.jpg"
        }
      },
      {
        "type": "video"
        "video": {
          "id": 12
          "uid": "XCSSMDFWW"
          "source": "youtube"
        }
      }
    ]
  }
}
```

When embedding ids:

```ruby
class PostSerializer < ActiveModel::Serializer
  embed :ids

  attributes :id, :title
  has_many :attachments, polymorphic: true
end
```

```json
{
  "post": {
    "id": 1,
    "title": "New post",
    "attachment_ids": [
      {
        "type": "image"
        "id": 12
      },
      {
        "type": "video"
        "id": 3
      }
    ]
  }
}
```